Step polymerization dendritic polymer

The solid-phase synthesis of dendritic polyamides was explored by Frechet et al. [49]. Inspired by the technique used by Merrifield for peptide synthesis, the same strategy was used to build hyperbranched polyamides onto a polymeric support. The idea was to ensure the preservation of the focal point and to ease the purification between successive steps. The resulting polymers were cleaved from the solid support, allowing ordinary polymer characterization. The reaction was found to be extremely sluggish beyond the fourth generation. 

Ultrafiltration has been used for the separation of dendritic polymeric supports in multi-step syntheses as well as for the separation of dendritic polymer-sup-ported reagents [4, 21]. However, this technique has most frequently been employed for the separation of polymer-supported catalysts (see Section 7.5) [18]. In the latter case, continuous flow UF-systems, so-called membrane reactors, were used for homogeneous catalysis, with catalysts complexed to dendritic ligands [23-27]. A critical issue for dendritic catalysts is the retention of the catalyst by the membrane (Fig. 7.2b, see also Section 7.5). 

Dendritic polymers with enhanced amplification and interior functionality were prepared by Tomalia et al. (3) by slow step-growth polymerization techniques including polyimine formation followed by rapid ring-opening reactions. 

Because of the one-step polymerization procedure, hyperbranched polymers often contain not only D and T but also L repeating units. This can be expressed by DB, which is an important structural parameter of hyperbranched polymers. DB is estimated as the sum of the D and T units divided by the sum of all the three structural units, that is, D, T and L [41]. By definition, a linear polymer has no dendritic units and its DB is zero, while a perfect dendrimer has no linear units and its DB is thus unity. Frey has pointed out that DB statistically approaches 0.5 in the case of polymerization of AB2 monomers, provided that all the functional groups possess the same reactivity [42]. The structures of the hb-PYs could be analyzed by spectroscopic methods such as NMR and FTIR. The DB value of the phosphorous-containing polymer hb-F21, for example, was estimated to be 53% from its 31P NMR chemical shifts (Chart 1). 

Figure 5.25. Illustration of resultant polymers through varying the degree of control of step-growth polymerization. Each successive growth layer is referred to as a generation (G). Reproduced with permission fromFrechet, J. M. J. Tomalia, D. A. Dendriniers and Dendritic Polymers, Wiley New York, 2001.

Our research interest in this field is based on the perception that these dendritic polymers could be useful as polymer-rheology control agents as well as spherical, multifunctional macromonomers. Hyperbranched polymers, which were not only thermally and chemically robust under the conditions used, but also could be economically obtained, were created to evaluate these concepts. The latter requirement led us to pursue the one-step polymerization of AB -type monomers. We will review mostly the synthesis of aromatic polymers with stable chemical linkages prepared by the single-step direct method, and we will briefly compare them with polymers made by more controlled multistep syntheses. 

Many methods have been reported to synthesize hyperbranched polymers. These materials were first reported in the late 1980s and early 1990s by Odian and Tomalia [9], Kim and Webster [10], and Hawker and Frechet [11]. As early as 1952, Hory actually developed a model for the polymerization of AB -type monomers and the branched structures that would result, identified as random AB polycondensates [46], Condensation step-growth polymerization is likely the most commonly used approach however, it is not the only method reported for the synthesis of statistically branched dendritic polymers chain growth and ringopening polymerization methods have also been applied. 

Analogous chemistry (i.e., end functionalization of polymeric organolithiums with l,l-bis(4-tert-butyldimethylsi-loxyphenyl)ethylene) has been utilized for preparation of dendrimacs, a new type of dendritic polymer wherein a key step in the iterative process utilizes the deprotected phenol end groups in polymers as nucleophiles for Williamson ether-type coupling reactions. Hirao and co-workers " have utilized di-monosaccharide-substituted DPEs to prepare well-defined polyisoprenes and polystyrenes labeled at the chain end with monosaccharide residues. 

The field of step-growth polymers encompasses many polymer structures and polymerization reaction types. This chapter attempts to cover topics in step-growth polymerization outside of the areas reviewed in the other introductory chapters in this book, i.e., poly(aryl ethers), dendritic polymers, high-temperature polymers and transition-metal catalyzed polymerizations. Polyamides, polyesters, polycarbonates, poly(phenylene sulfides) and other important polymer systems are addressed. The chapter is not a comprehensive review but rather an overview of some of the more interesting recent research results reported for these step-growth polymers, including new polymerization chemistries and mechanistic studies. 

In order to prepare a hyperbranched dendritic polymer by chain polymerization, a new concept of pol)mierization, termed multibranching polymerization (MBP) was proposed, which provides a dendritic polymer involving the initiator as the core [23] The most characteristic aspect of MBP is multiplication of the propagating ends at every step of propagation according to Scheme 5 ... 

Hyperbranched polymers are characterized by their degree of branching (DB). Hie DB of polymers obtained by the step-growth polymerization of AB2-type monomers is defined by Eq. (2.1) in which dendritic units have two reacted B-groups, linear units have one reacted B-group, and terminal units have two unreacted B-groups191 ... 

The polymerization of AB -functional vinyl monomers is fundamentally different from the step-growth polymerization of AB2-monomers. Condensation of AB2-monomers results immediately in the formation of hyperbranched polymers since the reactivity of the end-groups are the same, regardless of what type of repeat unit (linear or dendritic) that is formed. 

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